Underwater gun systems are being developed for naval warfare. These systems often use an energetic propellant to launch a projectile from a launch tube. A challenge to the development of effective underwater guns is that a projectile traveling through water experiences a resistance or drag that is approximately one thousand times greater than the resistance experienced by the projectile traveling through air. As a consequence of this high level of drag, conventional underwater projectiles are limited to speeds of no more than about 80 kilometers/hour (km/h).
The high resistance presented by the water medium can be addressed via a phenomenon known as “supercavitation.” This phenomenon can occur when a projectile having a blunt nose travels at sufficiently high speeds under water. The blunt nose pushes aside water as the projectile advances. When the hydrodynamic pressure of water that is pushed aside overcomes the ambient static pressure, water vaporizes. The vaporized water forms air bubbles, which coalesce to form a “cavity” in the water. If enough bubbles are formed, the cavity will be large enough to completely engulf the projectile, with the exception of the blunt tip of the nose. This characterizes the supercavitating mode of operation, which is also referred to as “cavity-running” operation).
Within the vaporous cavity, the projectile is effectively traveling through air rather than water. The projectile, therefore, experiences greatly reduced drag. As a consequence, the projectile is capable of attaining a velocity far in excess of what is possible when traveling through water proper.
Supercavitating projectiles often collide with the walls of the enveloping cavity, which increases drag. This can be addressed by equipping the projectile with fins. When a fin contacts the cavity wall, a torque develops that steers the projectile toward the center of the cavity into a region of lower drag.
The fins are usually located in the aft section of the projectile body and project radially outward therefrom. The radially-extending fins prevent the projectile from being tightly packaged within a launch tube. This drawback is addressed by coupling the projectile to a sabot, which is a carrier that centers the projectile within the launch tube and falls off after launch. Use of a sabot disadvantageously increases the amount of energetic propellant required for launch and also requires an increase in launcher size. A need therefore exists for an improved supercavitating projectile that retains the in-cavity stability of known fin designs but does not require a sabot for launch.